Effector CD8+ T cells in HIV infection are responsible for initial viremia control, however, HIV-specific CTL memory cells are lost during disease progression by an unknown mechanism. Our data show that HIV+ patients contain HIV-specific CD8+ CD28dim cells which die by costimulation via monocytes. In addition, we found that HIV causes over-expression of the CD28 ligands (CD80 and CD86) in vitro, inducing CD28 loss and apoptosis of CD8+ T cells. These observations suggest a fundamental mechanism whereby CD8+ T cells capable of becoming memory HIV-specific CTL could be selectively lost during disease progression. To study this hypothesis we propose three aims. Aim 1 examines mechanisms for CD28 down-regulation and apoptosis. Using mixtures of CHO cell lines with defined levels of costimulatory ligands, CD80 or CD86, incubated with purified T cells, we will assess mechanisms responsible for Cd28 down-regulation and apoptosis. To examine the importance of HIV infection, we will mix autologous HIV-infected macrophages with T cells, examine CD80 and CD86 expression on the macrophages, as well as the effects of cell contact, soluble factors or cytokines released by the macrophages on CD28 down-regulation and apoptosis of the CD8+ T cells. Because little is known about molecular regulation of CD28 expression, Aim 2 will examine the down-regulation of the CD28 promoter by testing differential binding of NF-kBalpha and/or STAT-6 proteins, which will provide a molecular basis for prevention of CD28 loss. Aim 3 will develop potential therapeutic methods to increase antigen-specific memory CD8+ T cells in HIV. Using alloantigen as a model, we will modulate costimulation, activation conditions, timing, strength of signals, and cytokines. The most promising methods to increase memory CD8+ T cells will be used with CD8+ T cells from HIV-infected patients. Because CD8+ T cells are important for HIV control, these studies are important for development of immune intervention strategies and for vaccine design.